Modular multicell batteries represent a total departure from traditional designs for SLI automotive lead-acid batteries. Traditional designs incorporate a premolded container divided into individual cell compartments. Battery plates and separators are separately formed and are inserted into the cell compartments during assembly of the battery.
Modular battery designs, such as those disclosed in U.S. Pat. Nos. 4,239,839 and 4,209,579 to W. McDowall et al. and U.S. Pat. Nos. 4,022,951 and 3,941,615 to W. McDowall, however, do not have a premolded container. Instead, the container, such as it is, is formed by assembling the battery plates and separators themselves, a process which is made possible by the unique structure of the plate and separator assemblies used in modular batteries.
Traditionally designed lead-acid, SLI batteries have plates consisting of a grid-like alloy current collector which is layered with electrochemically active paste to form a plate. The plates in modular batteries, however, are fabricated from collector assemblies. The collector assemblies have a lead alloy current collector, usually in the form of a mesh or sheet, which is molded into a thermoplastic frame. The thermoplastic frame has various dividers which define a number of areas into which positive and negative paste is applied. The separator assemblies are similar, having separator material molded into a thermoplastic frame of like construction.
After the collector assemblies are pasted to form plates, the plates are stacked along with separator assemblies, and the perimeter members of the thermoplastic frames are welded together to create a leak-tight container. Likewise, dividers in adjacent frames are welded together at the same time to isolate adjacent positive and negative areas of the plates.
The only method commercially utilized to paste modular battery collectors uses a pasting machine similar to that disclosed in U.S. Pat. No. 4,307,758 to W. McDowall et al. which is designed to paste collector assemblies having a mesh current collector. That pasting machine incorporates a number of paste extrusion nozzles communicating with a paste reservoir. When the machine is being operated, flow of paste through the extrusion nozzles is controlled by sliding gate valves mounted over the nozzle orifices. Each nozzle is mounted, and its associated valve actuated, so that discrete paste strips are extruded from a nozzle orifice and laid on a collector assembly in each of its paste areas. After the extruded paste strips are laid over the collector assembly, the collector assembly is conveyed downline to set the paste. The paste is set in the collector assembly by ultrasonic horns which vibrate the paste, forcing the paste to settle through the mesh current collector.
While this machine has various advantages, especially as compared to conventional pasting machines which are wholly unsuited for pasting collector assemblies for modular batteries, it suffers from a number of deficiencies. One such deficiency relates to finish control over the paste which is applied to a collector assembly. That is, the amount, thickness, uniformity, and coverage of finished paste often is unacceptable and is difficult to control in a reliable and reproducible manner. Such finish control problems arise from a number of sources and can lead to various performance-related problems.
The thickness of the paste layer applied to a collector assembly by the McDowall machine is controlled, at least in theory, by varying the thickness of the strip extruded from the nozzle, which in turn is controlled by varying the dimensions of the nozzle orifice. Apart from the various finish control problems inherent elsewhere in the process, as discussed below, sizing the paste layer by varying orifice dimensions in an extrusion nozzle can be troublesome. Especially when thinner strips are extruded, it has been observed that the extrusion nozzle has a tendency to clog. Thus, the machine suffers significant downtime while the nozzle is removed and cleaned.
Moreover, a paste strip, as it is ultrasonically vibrated in the McDowall machine, tends to penetrate through the collector mesh unevenly. Paste tends to accumulate in central areas of the mesh, frequently to the extent that perimeter areas of the collector are exposed. Ultrasonic vibration also causes localized, variation in thickness, referred to as "pebblestoning" for its visual resemblance to its namesake. Ultrasonic vibration also can cause some of the paste strip to penetrate too far through the mesh current collector, to the extent that it creates unwanted projections from the other face of the plate. It is necessary to "denib" the paste surface; and this removes a certain, but unpredictable, amount of paste from each collector assembly. This lack of uniform amounts, thickness, and coverage of paste causes uneven current distribution from the completed plate, and exposed portions of the mesh current collector are subject to corrosion by sulfuric acid electrolyte.
Moreover, because the finish control is unreliable, paste often is applied to a level beyond the surfaces of the frame. This means that the pasted collector assemblies must be dried before they can be stacked. Otherwise, the paste will cause adjacent stacks to stick together, and plates will be damaged upon separation.
The alignment of the extrusion nozzle relative to the pasting area of a collector assembly also is subject to variation. Such misalignment problems, as well as ultrasonic vibration, can cause paste to be spread on the frame where obviously no paste should be present. As a practical matter, therefore, each collector frame must be cleaned prior to welding a battery stack together. Otherwise, it is very likely that a battery so formed will leak.
The plates are cleaned by mechanical action, and that process often damages the plates, causing waste and scrap. Cleaning the plates also removes a certain amount of paste, which will vary from plate to plate, further diminishing the finish control of the process. Drying of the plates is a prerequisite for cleaning as well.
Drying entails passing the pasted collector assemblies through a tunnel oven. The temperature of the oven must be kept relatively low, however, to prevent melting of the thermoplastic frames. The drying time, therefore, must be extended by reducing the speed at which pasted collector assemblies are passed through the oven as compared to the higher speeds and outputs in oven-drying of traditional battery plates. Consequently, the oven-drying stage considerably slows the overall output of a pasting line using the McDowall machine: outputs of about 40 grids/minute as compared to outputs of 120 grids/minutes or more for other types of pasting machines.
Cleaning, denibbing, and vibrating the paste applied to collector assemblies also generate lead dust. This lead dust may create a potential hazard and may have to be removed from the air at significant expense and effort.
Finally, the McDowall machine is poorly suited to paste collector assemblies which have an unperforated metallic sheet as the current collector since it is designed to ultrasonically force the extruded paste strip through a mesh current collector. Pasting a collector assembly with a foil current collector would require passing each collector assembly through the McDowall machine twice. Moreover, pasting a foil collector would require that thinner strips be extruded from the extrusion nozzles which, as noted, can lead to more frequent clogging of the nozzle. It also is difficult to obtain intimate electrical contact between the extruded paste strip and the foil collector.
An object of this invention, therefore, is to provide methods and apparatus for pasting battery current collectors, especially modular battery current collectors, which are faster than conventional methods and apparatus for pasting modular current collectors.
It also is an object to provide such pasting methods and apparatus which provide better finish control of paste applied to a current collector as compared to conventional methods which may produce unevenness and poor coverage control.
Another object of this invention is to provide pasting methods and apparatus which reduce waste caused by damage to the plates during cleaning of the frame as compared to methods and apparatus which rely on physical cleaning of frames that are contaminated with paste as a collector assembly is pasted.
A further and related object is to provide such methods and apparatus which minimize or even eliminate the need to clean the current collector frames after pasting.
It is a further object of this invention to provide current collector pasting methods and apparatus which generate less lead dust than methods and apparatus which rely on cleaning, denibbing, and ultrasonically vibrating paste applied to a collector assembly.
Yet another object of this invention is to provide current collector pasting methods and apparatus which reduce the amount of downtime caused by clogging of nozzles.
A further object is to provide such methods and apparatus which more efficiently paste collector assemblies having a current collector which is an unperforated metallic sheet.
It also is an object of this invention to provide current collector pasting methods and apparatus wherein all of the above-mentioned advantages are realized.
Those and other objects and advantages of the invention will be apparent to those skilled in the art upon reading the following detailed description and upon reference to the drawings.